Toxoplasma gondii infections cause suffering and mortality in those congenitally infected or immune compromised. Easily genetically manipulated and disseminated in nature, this parasite is considered a category B bioterrorism pathogen. Improved, new antimicrobials are greatly needed to treat this infection. Type 2 fatty acid biosynthesis (fas), structurally different from mammalian type 2 fas, is a validated target essential for Toxoplasma gondii growth in vitro and survival of the parasite in mice. Enoyl reductase (ENR) is an enzyme in the type 2 fas biosynthetic pathway. Triclosan and two newly synthesized lead compounds, in nanomolar amounts, inhibit T. gondii ENR activity and in low micromolar amounts inhibit T. gondii growth in vitro. Triclosan also inhibits T. gondii growth in mice. Structures of T. gondii ENR complexed with inhibitors have been solved. Unique features of T. gondii ENR will be used to develop novel tight binding inhibitors. With these data in hand, and to be acquired during the work proposed, using a structure based design approach, it will be possible to modify and thereby optimize and create a novel tight binding inhibitor of T. gondii ENR. We will create a new compound based on theoretical information derived from analyses of lead inhibitory compounds'and ENR structures and their interactions coupled with enzyme and parasite inhibitory data. Information we have in hand from our preliminary analyses of triclosan and other novel compounds we have synthesized and tested in enzyme, in vitro parasite and co-crystallization and structure analyses will inform this structure-based ENR inhibitor design program. If new leads are needed, as an additional alternative approach, a Catalyst-based pharmacophore approach will be used. Inhibitors will undergo rounds of modification and testing to find more druggable ENR inhibitors with better ADMET profiles. As new inhibitory compounds are identified, their fit to the enzyme binding site will be compared, and hybrid structures that incorporate desirable features from different molecular scaffolds will be created to optimize activity. ClogP, or PSA, and Caco2 cell permeability will be optimized. It is likely that either a completely new molecule will be created or that the phenyl rings of triclosan will be converted to heterocycles having improved solubility, activity, and safety. RELEVANCE (See instructions): Development of inhibitors useful as medicines will be progressed by modification of lead compounds to obviate potential problems with resistance and toxicity and optimize solubility, pharmokinetics, and bioavailability. This work will create a greatly needed novel class of compound to treat toxoplasmosis.